The present invention relates to a control circuit for regulating fluid communication between a pump or source and a hydraulic motor. More particularly, the invention relates to such a circuit adapted for regulating high pressure fluid flow as is commonly required in the operation of various machines such as earth-moving equipment.
In circuits of this type, load pistons subject to variable loading are commonly employed to modulate fluid pressure in an inlet chamber of a control valve during operation of the motor. With the control valve being of the closed-center or other types tending to block the inlet chamber when the motor is not being operated, pressurized fluid entering the inlet chamber from a source must constantly be vented or returned to a reservoir providing a fluid supply for the source. The load piston may be employed for this function.
However, in the prior art circuits, the load piston remains subject to substantial loading even when the motor is not operating. Accordingly, the pressure in the inlet chamber is relatively high even when the control valve is in neutral and the motor is not operating, causing substantial heat generation and power consumption when the fluid is vented from the inlet chamber to drain against the substantial force acting on the load piston.
Further, the substantial pressures remaining in the inlet chamber create flow forces acting on movable portions of the control valve. In high pressure circuits, the resulting flow forces may tend to hydraulically lock the control valve spool.
These problems are most severe in circuits employing high fluid pressures, for example, operating pressures in the order of 3,500 psi. Under such conditions, the flow forces acting upon the control valve spool are commonly so great that the spool cannot readily be manually operated but must rather be operated by pilot fluid pressure sufficient to counter the flow force effect on the spool. Even with pilot-operated control valves, the substantial pilot pressures required for operation of the spool make it difficult to provide accurate modulated control over fluid pressure in the inlet chamber. The lack of accurate pressure modulation may tend to cause erratic operation of the hydraulic motor which would be particularly noticable, for example, where accurate positioning of a load is to be accomplished by the motor.
Accordingly, it is one object of the present invention to provide a control circuit for minimizing or eliminating one or more of the problems referred to above.
It is also an object of the present invention to provide a control circuit for two hydraulic motors, each having a control valve, wherein a bypass valve conditions one of the control valves to provide pressure modulation during operation of the other control valve.